Superplastic metallic alloys, such as for instance certain fine grain alloys of aluminum, magnesium, stainless steel and titanium, are relatively ductile materials that can undergo substantial tensile deformation in the presence of low shaping forces. After being heated to a suitable forming temperature, these materials become capable of being stretched and formed over a forming tool and/or into a die cavity to make complex shaped parts, e.g., automotive body parts, or the like. This process often is referred to as superplastic forming.
In the superplastic forming (SPF) process, a sheet metal blank is positioned with one side lying close to the hot forming surface of a heated forming tool in a press. The metal sheet is often preheated to its forming temperature, and is gripped at its peripheral edges between complementary opposing dies. A pressurized fluid, such as for instance air, is applied to the other side of the sheet metal blank, thereby forcing and stretching it into conformance with the forming surface of one die while at the same time maintaining a target strain rate for deforming the sheet throughout the forming cycle. The superplasticity of the material enables forming of complex components that cannot be formed by conventional room temperature metal forming processes. For instance, use of the SPF process enables the forming of a workpiece with a deep cavity or with a cavity that is formed over very small radii. Further, superplastic forming often permits the manufacture of large single parts that cannot be made by other processes, such as for instance sheet metal stamping. In fact, a single part that is formed using the SPF process can sometimes replace an assembly of several parts that are made from non-superplastic forming materials and processes.
Due to the high pressures and temperatures that are employed in the SPF process, special attention must be paid to creating an effective gas-tight seal between the surfaces of the forming apparatus and the sheet metal blank. Typically, the upper and lower dies of the forming apparatus are moved together such that they press in opposite directions against the peripheral edges of the sheet metal blank. Specialized sealing features are often located on the dies in order to ensure that adequate sealing is achieved even when the temperature of the forming apparatus is raised and the shape of the dies may change. In contrast, forming processes that are carried out at lower temperatures typically do not require elaborate measures to ensure a gas-tight seal, since the tool parts are not heated to temperatures that are high enough to induce shape changes therein.
A common feature of many of the known SPF systems is that only one sheet metal blank at a time undergoes superplastic forming. In these systems, the pressurized gas is introduced via a passageway that is defined through one tool half, so as to cause the sheet metal blank to stretch and conform to the heated forming surfaces of the other tool half. This arrangement facilitates the formation of a gas-tight seal all the way around the periphery of the sheet metal blank, such that gas leakage is readily prevented. Unfortunately, the SPF process has a relatively long cycle time. Further, a considerable amount of energy is required in order to maintain the forming dies at the SPF process temperature. The combination of long cycle time and high energy usage makes it considerably more expensive to form parts using the SPF process compared to other processes, and therefore the SPF process has generally been limited to low volume and/or high value applications. That being said, the SPF process could be used to good advantage in a wide variety of other applications, if the low production rate and high cost issues are resolved.
One approach that has been investigated involves the simultaneous forming of two sheet metal blanks so as to produce two parts during each SPF cycle. Optionally, the two parts are identical or the two parts are different. This approach not only increases the part production rate, but it also reduces the amount of energy that is consumed in heating the dies on a per-part basis. Unfortunately, the simultaneous forming of two sheet metal blanks generally requires a more complex system for introducing the pressurized gas. In particular, it is necessary to introduce the gas into a region between the facing surfaces of the sheet metal blanks, while at the same time creating and maintaining a peripheral gas-tight seal between the two sheet metal blanks, even under conditions of high temperature and high internal pressure.
U.S. Pat. No. 6,694,790 in the name of Ryntz et al. discloses a system for the superplastic forming of parts from plural sheets. Ryntz et al. teaches a mid-plate assembly, in which a frame-shaped mid-plate is disposed between two blanks in a forming die. During use the mid-plate spaces apart the two blanks, so as to create a cavity therebetween. A lower tool having a sheet-piercing nozzle is configured such that the nozzle seats into the mid-plate to form a gas connection for supplying pressurized gas into the cavity between the blanks via a passageway that is defined through the mid-plate. This system is somewhat complicated and requires the use of a cumbersome mid-plate in addition to the standard components of a traditional superplastic forming apparatus. Further, the presence of the mid-plate creates an additional interface that must be sealed gas-tight, and there are additional maintenance issues relating to the sheet-piercing nozzle, etc.
U.S. Pat. No. 6,675,621 in the name of Kleber discloses another system for the superplastic forming of parts from plural sheets. According to Kleber, forming dies are moved to a closed position on each of a pair of stacked blanks so that a partial perimeter gas seal is established therebetween. A pressure wedge is then introduced between the two blanks along one edge of the pair, so as to act as a stopper or air seal to complete perimeter sealing. The pressure wedge also establishes the operative position of a gas injection port, which directs pressurized air interiorly of the completed perimeter seal of the pair of blanks. However, this system requires the use of unequal, oversized sized blanks and it does not appear to be readily adaptable to forming parts of different widths. Further, the system disclosed by Kleber does not appear to address formation of a gas-tight seal at the edges of the pressure wedge.
It would be beneficial to provide a system and method for forming composite articles from prepregs, which overcome at least some of the above-mentioned limitations of the prior art.